Method and apparatus for detecting defects on a wafer

ABSTRACT

A light is irradiated on a wafer including a plurality of pixels. Image information corresponding to each pixel is measured by sensing the light reflected by the wafer surface. A raw datum is calculated by subtracting the image information of a corresponding pixel from the image information of a target pixel. The target pixel is a subject pixel for detecting a defect. The corresponding pixel is a pixel located in a first device unit and corresponds to the target pixel. The first device unit is located adjacent to a second device unit that includes the target pixel. The threshold region is preset to have at least one pair of upper and lower limits. The target pixel is marked as a defective pixel when the raw datum thereof is included in the threshold region. Accordingly, the killer defect can be detected separate from the non-killer defects that are usually detected together with the killer defects.

CROSS-REFERENCE OF RELATED APPLICATIONS

[0001] This application claims priority upon Korean Patent ApplicationNo. 2003-00102 filed on Jan. 2, 2003, the contents of which are hereinincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method and an apparatus fordetecting a defect on a wafer.

[0004] 2. Description of the Related Art

[0005] In general, fine patterns of semiconductor integrated circuitsformed on a wafer are inspected for detecting pattern defects afterperforming a semiconductor fabrication process step or a series ofsemiconductor fabrication process steps. As semiconductor devices becomemore highly integrated and a diameter of the wafer becomes larger, theinspection process for detecting defects on the wafer is more frequentlycarried out. Therefore, overall manufacturing time for manufacturing thesemiconductor device has been significantly increased, thereby raisingthe manufacturing cost of the semiconductor devices.

[0006] Conventionally, an individual gray level corresponding to each ofpixels on the wafer is measured, and the gray level of a target pixeland the gray levels of neighboring pixels adjacent to the target pixelare compared with each other. Then, the gray level difference iscalculated. The inspection process detects defects of the wafer by usingthe gray level difference. The inspection process is classified into anarray mode and a random mode. While the array mode compares respectivecells in a chip on the wafer for detecting defects, the random modecompares respective die for detecting defects. The array mode is usuallyused in semiconductor memory device fabrication process, and the randommode is usually used in logic device fabrication process. Hereinafter,the inspection process for detecting defects will be explained for thearray mode.

[0007] In general, the widespread array mode inspection process uses athreshold value for detecting defects on the wafer. The gray leveldifference between the target pixel and the neighboring pixels adjacentto the target pixel is compared with a preset threshold value. When thegray level difference is greater than the preset threshold value, thetarget pixel is indicated as a defective pixel. On the contrary, whenthe gray level difference is less than the threshold value, the targetpixel is indicated as a non-defective pixel.

[0008]FIG. 1 is a schematic diagram showing a conventional inspector.Referring to FIG. 1, a wafer 12 on which predetermined process stepshave been carried out is loaded on a support 14 for detecting processdefects on the wafer. The wafer 12 is loaded/unloaded to/from thesupport 14 by the conventional loading mechanism such as a robot arm. Alight source 10 irradiates a light to each cell on a surface of thewafer 12. Then, the light is reflected from the surface of the wafer 12.The reflected light is detected by an image detecting means 16 includinga photo-sensor, and as a result, an analog image signal is generated.The analog image signal is converted into a digital image signal by ananalog-to-digital converter (ADC). Thus, gray levels corresponding torespective pixels comprising each cell on the wafer are formed. The graylevel is processed with 8-bit digital signal, so that each pixel of thecell may have 256 kinds of gray levels. Therefore, gray levelscorresponding to every pixel form the digital image corresponding to acell on the wafer, and all the digital images corresponding to everycell forms an image map corresponding to one sheet of the wafer. Then, adata process unit 20 generates a raw datum. The raw datum is a graylevel difference between the gray level of the target pixel and the graylevel of the neighboring pixel adjacent to the target pixel. On theother hand, a threshold presetting unit 24 presets a threshold value,which is used for judging whether a defect is formed on the wafer. Theraw datum is calculated into an absolute value, and is compared with thethreshold value. The defect on the wafer is detected using a detectingunit 22. The detecting unit 22 includes a central process unit (CPU) anda co-processor, and detects the defect on the wafer by using a mainprogram and a sub-program. The result of the detecting unit 22 isdisplayed on a monitor of the operating terminal 26.

[0009]FIG. 2 is a diagram explaining the generation of the raw datum bythe data process unit shown in FIG. 1.

[0010] Referring to FIG. 2, a light is irradiated on a first cell A thatis an arbitrary cell on the wafer at an arbitrary time to. A first imageI₁ corresponding to the first cell A is obtained. Next, a light isirradiated on a second cell B adjacent to the first cell at a time t₀₊₁after a lapse of unit time from the time t₀, and a second image I₂corresponding to the second cell B is obtained. A third imagecorresponding to the third cell, a fourth image corresponding to thefourth cell, and a fifth image corresponding to the fifth cell aresequentially obtained in the same manner. Thus, the image mapcorresponding to an entire surface of the wafer is obtained. The size ofthe cell is determined such that the same pattern is repeated for everyimage obtaining step. Each of the images is represented as the graylevel of the pixels comprising the respective cells on the wafer, andthe gray level is binary digital data. Therefore, an image differenceI₁-I₂ between the first and second images is the binary digital datum.

[0011]FIG. 3 is a diagram explaining a process for detecting the defectson the wafer by using the detecting unit shown in FIG. 1. FIG. 3 showsarbitrary 3 cells neighboring each other on the wafer for simplicity.The same alphabetic letter indicates a pixel located on the sameposition on different cells, and the same numeric letter indicates anidentical cell.

[0012] An experiment shows that the gray levels of each pixel B1, B2,and B3 of FIG. 3 are 50, 100, 50, respectively, and the gray levels ofeach pixel C1, C2, and C3 of FIG. 3 are 60, 30, 60, respectively. Thatis, the B2 pixel is more luminescent than the B1 and B3 pixels, and theC2 pixel is less luminescent than the C1 and C3 pixels. The raw datum ofthe B2 pixel is the gray level difference between the B2 pixel and theadjacent pixels B1 and B3. That is, the raw datum of the B2 pixel iscalculated as the gray level difference of (gray level of B2 pixel−graylevel of B1 pixel) and (gray level of B2 pixel−gray level of B3 pixel).In the same way, the raw datum of the C2 pixel is calculated as the graylevel difference of (gray level of C2−gray level of C1) and (gray levelof C2−gray level of C3). According to the present experiment, the rawdatum of the B2 pixel is 50, and the raw datum of the C2 pixel is −30.The negative raw datum is converted into the same positive value byconverting into an absolute value. When the threshold value is 40, theB2 pixel is checked as a defective pixel and the C2 pixel is checked asa non-defective pixel.

[0013]FIG. 4 is a flow chart illustrating a conventional method ofdetecting a defect on the wafer.

[0014] Referring to FIG. 4, a light is irradiated on a surface of thewafer on which a thin film is deposited, and in step S10, gray levels ofeach pixel on the wafer are formed. In next step S20, the raw datum thatis the gray level difference between the target pixel and theneighboring pixel is generated, and in step S30, the threshold valuethat is a criterion for judging defectiveness after compared with theraw datum is preset. In step S40, the raw datum is checked whether ornot the value is negative. When the raw datum has a negative value, theraw datum is converted into the positive value by using the absolutevalue of the negative raw datum in step S42. In subsequent step S50, theraw datum is compared with the threshold value, and when the raw datumis more than the threshold value, the target pixel is checked as adefective pixel in step S60.

[0015] However, the conventional detecting method has the followingproblems:

[0016] First, a killer defect, meaning a serious defect, and anon-killer defect, meaning a non-serious defect, are simultaneouslydetected because all of the target pixels of which the absolute value ofthe raw datum is more than the threshold are checked as a defectivepixel. Therefore, the killer defect and the non-killer defect are noteasily separated from each other.

[0017] A process of after-develop inspection (ADI) is carried out on asurface of the wafer on which 0.09 mm design rule is applied fordetecting a micro-bridge that is a killer defect found on a gate of anon-versatile memory (hereinafter, referred to as NVM). According to theADI data measured by the STEALTH (trade name, a detecting apparatus madeby KLA-Tencor Co. Ltd. U. S. A.), about 50% of the detected defects weresub-defect that is a common defect found under the layer, and about 40%of the detected defects were a non-visual defect such as the falsedefect that is not visible through the scanning-electron microscope(SEM) and caused by the interference of the incident light or theoperating error of detecting apparatus. That is, about 90% of thedetected defects were non-killer defects.

[0018]FIG. 5 is a diagram showing the gray level difference with respectto the defect type detected on the NVM. The horizontal axis of thediagram indicates the defect type, and the vertical axis of the diagramindicates the gray level difference.

[0019]FIG. 5 confirms that the gate level differences of the sub-defectand the false defect are greater than the gate level difference of thebridge defect that is the killer defect generated during the NVMfabricating process. Therefore, when the threshold value is exemplarilypreset as about 30 for detecting the bridge defect, the sub-defect andthe false defect are also detected with the bridge defect. Therefore,the bridge defect is not separated from the sub-defect and the falsedefect.

[0020] Second, when the negative raw datum is converted into a positivevalue by using an absolute value of the negative raw datum, anoriginally positive raw datum is not distinguished from the convertedpositive raw datum. Therefore, the defect corresponding to theoriginally positive raw datum and the defect corresponding to theconverted positive raw datum are simultaneously detected, so that thedetected defect cannot be verified.

[0021] An inspection process is carried out on a surface of the wafer onwhich 0.123 mm design rule is applied after patterning process forforming a S-poly (storage node with polysilicon), and the respectivegray levels of the detected defects are measured. The inspectiondiscloses not only, for example, a striation defect which is a commondefect of the S-poly patterning process, and the false defect, but alsoa leaning defect that is a killer defect of the S-poly patterningprocess. The false defect is not visible through the scanning-electronmicroscope (SEM), and it is caused by the interference of the incidentlight or the operating error of detecting apparatus.

[0022]FIG. 6 is a diagram showing the gray level difference with respectto the defect type during the S-poly patterning process. The horizontalaxis of the diagram indicates an arbitrary position on the surface ofthe wafer, and the vertical axis of the diagram indicates the gray leveldifference.

[0023] Referring to FIG. 6, when the gray level difference is in a rangebetween about 20 and about 60, not only the leaning defect but also thestriation defect is most frequently detected. Therefore, when thethreshold value is preset as one of the range between about 20 and about60, the leaning defect and the striation defect are always detectedtogether.

[0024] The false defect is more frequently detected because the designrule is becoming smaller and smaller. Therefore, inspection time fordetecting the killer defect is getting longer, thereby reducing theproductivity of fabricating semiconductor devices.

SUMMARY OF THE INVENTION

[0025] Accordingly, the present invention is directed to a method and anapparatus for preventing various types of defects from being detectedtogether by using a threshold region including a pair of upper and lowerlimits that are compared with the raw datum of a target pixel.

[0026] According to one embodiment of the present invention, a method ofdetecting defects on a substrate includes irradiating a light on asubstrate, measuring image information on each pixel on the substrate,calculating a raw datum of a target pixel, presetting a thresholdregion, comparing the threshold region with the raw datum, and g)checking the target pixel as a defective pixel. Preferably, a pluralityof device units are formed on a surface of the substrate, and each ofthe device units includes a plurality of pixels. The image informationof a pixel is formed by sensing the light reflected by a surface of thesubstrate. The raw datum of the target pixel is formed by subtractingthe image information of a corresponding pixel from the imageinformation of the target pixel. The target pixel is a subject pixel fordetecting defects. The corresponding pixel is a pixel located in a firstdevice unit adjacent to a second device unit that includes the targetpixel. The corresponding pixel corresponds to the target pixel. Thethreshold region includes at least one pair of upper and lower limits.The target pixel is checked as a defective pixel when the raw datum iswithin the threshold region.

[0027] According to another embodiment of the present invention, anapparatus for detecting defects on a substrate comprises a support forsupporting a substrate, a light source for irradiating a light on thesubstrate, an image detector for sensing a light reflected from thesurface of the substrate, an analog-to-digital converter for convertingthe analog image information to a digital image information, a dataprocess unit for calculating a raw datum of target pixel, a settingmeans for presetting a threshold region, and a judging means for judgingwhether or not the target pixel is a defective pixel. A plurality ofdevice units are formed on a surface of the substrate, and each of thedevice units includes a plurality of pixels. The image detector formsanalog image information on each of the pixels. The raw datum of atarget pixel is formed by subtracting the image information of acorresponding pixel from the image information of the target pixel. Thetarget pixel is a subject pixel for detecting defects. The correspondingpixel is a pixel located in a first device unit adjacent to a seconddevice unit that includes the target pixel and corresponds to the targetpixel. The threshold region includes at least one pair of upper andlower limits. The judging means compares the raw datum of the targetpixel with the threshold region.

[0028] With the above exemplary embodiments, the killer defects can bedetected separate from the non-killer defects.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The above and other objects and advantages of the presentinvention will become readily apparent by reference to the followingdetailed description when considering in conjunction with theaccompanying drawings, in which:

[0030]FIG. 1 is a schematic diagram showing a conventional inspector;

[0031]FIG. 2 is a diagram explaining the generation of the raw datum bythe data process unit shown in FIG. 1;

[0032]FIG. 3 is a diagram explaining a process for detecting a defect onthe wafer by using the detecting unit shown in FIG. 1;

[0033]FIG. 4 is a flow chart illustrating a conventional method ofdetecting a defect on the wafer

[0034]FIG. 5 is a diagram showing the gray level difference with respectto the defect type detected on the NVM by using the conventionalinspector shown in FIG. 1;

[0035]FIG. 6 is a diagram showing the gray level difference with respectto the defect type during the S-poly patterning process by using theconventional inspector shown in FIG. 1;

[0036]FIG. 7 is a flow chart illustrating a method of detecting adefection a substrate according to an embodiment of the presentinvention;

[0037]FIG. 8 is a schematic diagram showing an apparatus for detecting adefect according to another embodiment of the present invention;

[0038]FIG. 9 is a graph illustrating a comparison between the thresholdregion according to an embodiment of the invention and the gray leveldifference according to the detected defect type; and

[0039]FIG. 10 is a graph illustrating the gray level difference withrespect to the defect type occurring during the S-poly patterningprocess, according to yet another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichexemplary embodiments of the present invention are shown.

[0041] Referring to FIG. 7, a light is irradiated on a surface of thesubstrate on which a predetermined process has already been carried outso as to detect defects in step S100. A plurality of device units formedon the substrate surface have the same pattern, and each of the deviceunits includes a plurality of pixels. As an exemplary embodiment, thesubstrate may be a wafer for fabricating a semiconductor device, and thedevice unit may be a unit cell operating as an independent electroniccircuit on the wafer. In addition, the light may have a relatively shortwavelength, which is referred to as a short-wave light. Therefore, thelight is relatively well reflected, hardly diffracts and hardlyinterferes on the substrate surface. For example, an ultraviolet lightmay be irradiated on the substrate surface.

[0042] In the following step S20, image information on each pixel isformed by the device unit through sensing the light reflected from thesubstrate surface. As an exemplary embodiment, the reflected light maybe received by a photo-sensor, and detected by a detecting unit. As aresult, analog image information on each of the pixels is formed. Theanalogue image information is stored for every device unit. The analogimage information may be converted into digital image information byusing the ADC. As an exemplary embodiment, the digital image informationmay be expressed as a gray scale distinguishable by a relative densityof black and white. The gray scale may be divided into 256 differentlevels by using an 8-bit microprocessor. Hereinafter, one of the 256levels of the gray scale is referred to as a gray level.

[0043] Subsequently in step S300, a raw datum of a target pixel isobtained by subtracting the gray level of a corresponding pixel from thegray level of the target pixel. The target pixel is a subject pixel fordetecting a defect. The corresponding pixel is a neighboring pixelpositioned in a first device unit adjacent to a second device unit thatincludes the target pixel corresponding to the target pixel.Accordingly, when the wafer is normally processed, the correspondingpixel has the same pattern as that of the target pixel even though thecorresponding pixel and the target pixel are positioned on differentcells of the wafer from each other, and as a result, the gray level ofthe corresponding pixel should be identical to the gray level of thetarget pixel. Therefore, the discordance of the target pixel gray levelwith the corresponding pixel gray level indicates that the target pixelis a defective pixel. The gray level is expressed as a binary digitsystem, and thus the difference between the gray levels is obtained bysubtraction of the binary digit indicating the gray level. According tothe present embodiment of the invention, the difference of the graylevels itself is used as the criterion for detecting a defect regardlessof subtraction result, i.e., positive or negative value of thedifference. Accordingly, a defect corresponding to a positive raw datumcan be prevented from being mistakenly detected together with a defectcorresponding to a negative raw datum.

[0044] A threshold region, which is a criterion for judgingdefectiveness of a target pixel, is then preset in step S400, and theraw datum is compared with the threshold region in step S500. Thethreshold region is defined by an upper limit and a lower limit, each ofwhich is a predetermined gray level difference. According to theconventional method of detecting defects as described above, the targetpixel is checked as defective pixel when the raw datum is greater thanthe threshold region. In contrast, according to the present embodimentof the invention, the target pixel is checked as a defective pixel onlywhen the raw datum is greater than the lower limit and smaller than theupper limit. Thus, when the raw datum is included in the thresholdregion, the target pixel is checked as a defective pixel. Finally, thedefective pixel is displayed on a monitor for visual observation, andthe defect type is visually verified.

[0045] Referring to FIG. 8, an apparatus for detecting a defect includesa support 140 for supporting a substrate on which a predeterminedprocess has already been carried out. For example, the substrate may bea wafer for fabricating a semiconductor having undergone a chemicalmechanical polishing (CMP) process, an etch-back process, a contactprocess, or an etching process.

[0046] A plurality of device units formed on the substrate surface havethe same pattern, and each of the device units includes a plurality ofpixels. When the substrate is a wafer for fabricating a semiconductordevice, the device unit is a unit cell operating as an independentelectronic circuit on the wafer. The conventional loading system such asa robot arm may be used for loading/unloading the wafer to/from thesupport. Hereinafter, the apparatus for detecting a defect according tothe present invention is exemplarily described with regard to the waferfor fabricating a semiconductor device and the cell on the wafer.However, the present invention is not limited to the wafer forfabricating a semiconductor device, as would be known to one of theordinary skill in the art.

[0047] A light is irradiated on a surface of the wafer 120 positioned onthe support 140 from the light source 100, and reflected from the wafersurface. An image detector 160 including a photo-sensor detects thereflected light, and generates analog image information for every pixelon each device unit.

[0048] An analog-to-digital converter 180 transforms the analog imageinformation into digital image information. As an exemplary embodiment,the digital image information may be expressed as a gray scaledistinguishable by a relative density of black and white. The gray scaleis divided into 256 different levels by using an 8-bit microprocessor,thus the pixel has a level among the 256 different levels, which isreferred to as a gray level corresponding to the pixel. Accordingly,when the gray level is obtained for every pixel of a cell, all of thegray levels constitute the digital image information of the cell as awhole. When the digital image information of each cell on the wafer isobtained, an image map for a wafer is formed.

[0049] A data process unit 200 generates a raw datum of a target pixelby subtracting the gray level of a corresponding pixel from the graylevel of the target pixel. The target pixel is a subject pixel fordetecting a defect. The corresponding pixel is a neighboring pixelpositioned in a first device unit adjacent to a second device unit thatincludes the target pixel corresponding to the target pixel.

[0050] A setting unit 240 presets a threshold region, which is acriterion for judging defectiveness of the target pixel. The thresholdregion is defined by an upper limit and a lower limit, each of which isa predetermined gray level difference.

[0051] A judging unit 220 compares the raw datum with the thresholdregion, and determines whether or not the target pixel is defective. Thetarget pixel is checked as a defective pixel when the raw datum isgreater than the lower limit and smaller than the upper limit. That is,when the raw datum is included in the threshold region, the target pixelis checked as a defective pixel. The judging unit 220 includes a centralprocess unit (CPU) and a co-processor, which is an auxiliary processorfor supporting the CPU during a numerical operation, and is operated bya main program and a plurality of sub-programs. Finally, the defectivepixel is displayed on a monitor for visual observation, and the defecttype is visually verified.

[0052]FIG. 9 is a graph illustrating a comparison between the thresholdregion according to an embodiment of the invention and the gray leveldifference according to the detected defect type. The various defectsshown in FIG. 9 are verified through the ADI process performed on awafer including the NVM gate as shown in FIG. 5. The horizontal axis ofthe graph indicates the defect type, whereas the vertical axis of thegraph indicates the gray level difference.

[0053] Referring to FIG. 9, as an exemplary embodiment, the thresholdregion for detecting the bridge defect, which is a killer defectgenerated during the NVM fabricating process, is defined by a lowerlimit that is identical to the conventional threshold value of 30 and byan upper limit above which a sub-defect and a false defect are detected.Accordingly, a sub-defect and the false defect are not detected sincethe gray level differences of a sub-defect and a false defect are morethan the upper limit. As a result, the above defined threshold regioncan only detect the bridge defect separate from the sub-defect and thefalse defect. That is, proper selection of the upper limit makes itpossible for the threshold region only to detect the killer defectseparate from non-killer defects that are usually detected together withthe killer defect. Therefore, the threshold region of the invention canimprove the accuracy and promptness of defect detection, therebyimproving the process efficiency.

[0054]FIG. 10 is a graph showing the gray level difference with respectto the defect type that occurs during the S-poly patterning process. Thehorizontal axis of the graph indicates an arbitrary position on thesurface of the wafer, and a vertical line of the graph indicates thegray level difference.

[0055] The graph illustrated in FIG. 10 shows that the leaning defect, akiller defect that occurs during the S-poly patterning process, is mostfrequently detected in a gray level difference range between about 20and about 60, and the striation defect, a non-killer defect during theS-poly patterning process, is most frequently detected in a gray leveldifference range between about −20 and about −60. According to theconventional detecting method using the absolute value of the raw datum,the leaning defect is detected together with the striation defect.However, according to the invention of detecting method using the rawdatum itself, the leaning defect is detected separate from the striationdefect. Accordingly, the apparatus for detecting a defect has advantagesin that the accuracy and promptness of defect detection is improved andthe process efficiency is also enhanced.

[0056] According to the present invention, the raw datum of the targetpixel is compared with the threshold region, and the target pixel ischecked as a defective pixel when the raw datum is included in thethreshold region. Therefore, the killer defect can more possibly bedetected separate from the non-killer defects that are usually detectedtogether with the killer defect. Thus, the accuracy and promptness ofthe defect detection is improved and the process efficiency is alsoenhanced.

[0057] Although the above exemplary embodiments discuss the method andapparatus for detecting a defect in the array mode, the method andapparatus disclosed could also be applicable in the random mode known toone of the ordinary skill in the art, since both of the array and therandom mode are based on the comparison so as to detect a defect on thewafer.

[0058] Although the exemplary embodiments of the present invention havebeen described, it is understood that the present invention should notbe limited to these exemplary embodiments but various changes andmodifications can be made by one skilled in the art within the spiritand scope of the present invention as hereinafter claimed.

What is claimed is:
 1. A method of detecting a defect on a substrate,the method comprising: irradiating a light on a substrate, wherein thesubstrate has a plurality of device units formed thereon with the samepattern, the plurality of device units each including a plurality ofpixels; measuring image information for the plurality of pixels bysensing the light reflected by a surface of the substrate from theirradiating light; calculating a raw datum of a target pixel bysubtracting the image information of a corresponding pixel from theimage information of the target pixel, wherein the target pixel is asubject pixel for detecting a defect, and wherein the correspondingpixel is located in a first device unit that is adjacent to a seconddevice unit that includes the target pixel, the corresponding pixelcorresponding to the target pixel; presetting a threshold regionincluding at least one pair of upper and lower limits; comparing thethreshold region with the raw datum; and marking the target pixel asdefective if the raw datum is within the threshold region.
 2. The methodof claim 1, wherein the substrate includes a wafer for fabricating asemiconductor device, and the plurality of device units are unit cellsoperating as independent electronic circuits on the wafer.
 3. The methodof detecting a defect on a substrate of claim 1, wherein the irradiatinglight includes a short-wave light.
 4. The method of detecting a defecton a substrate of claim 3, wherein the short-wave light includes anultraviolet light.
 5. The method of detecting a defect on a substrate ofclaim 1, wherein the image information includes binary digitalinformation.
 6. The method of detecting a defect on a substrate of claim5, wherein the image information represents a level on a gray scale,wherein the gray scale is distinguishable by a relative density of blackand white.
 7. The method of detecting a defect on a substrate of claim6, wherein the gray scale is divided into 256 different levels.
 8. Themethod of detecting a defect on a substrate of claim 1, furthercomprising displaying the defective pixel on a monitor.
 9. An apparatusfor detecting a defect on a substrate, the apparatus comprising: asupport for supporting a substrate, wherein the substrate has aplurality of device units formed thereon, each device unit including aplurality of pixels; a light source for irradiating a light on thesubstrate; an image detector for sensing a reflecting light reflected bya surface of the substrate from the light source; a data processing unitfor calculating a raw datum of a target pixel by subtracting digitalimage information of a corresponding pixel from digital imageinformation of the target pixel, wherein the corresponding pixel islocated in a first device unit that is adjacent to a second device unitthat includes the target pixel, the corresponding pixel corresponding tothe target pixel; a setting unit for presetting a threshold region,wherein the threshold region includes at least one pair of upper andlower limits; and a judging unit for judging whether or not the targetpixel is a defective pixel by comparing the raw datum of the targetpixel with the threshold region.
 10. The apparatus of claim 9, whereinthe substrate is a wafer for fabricating a semiconductor device and theplurality of device units are unit cells operating as independentelectronic circuits on the wafer.
 11. The apparatus of claim 9, whereinthe light in the light source includes a short-wave light.
 12. Theapparatus of claim 9, wherein the short-wave light includes anultraviolet light.
 13. The apparatus of claim 9, wherein the imageinformation is expressed as a gray scale distinguishable by a relativedensity of black and white.
 14. The apparatus of claim 9, furthercomprising a monitor for displaying a defective pixel and the raw datumof the defective pixel.
 15. The apparatus of claim 9, wherein the imagedetector includes a photo-sensor.
 16. The apparatus of claim 9, whereinthe image detector generates the analog image information for each pixelof each device units.
 17. The apparatus of claim 16, further comprisingan analog-to-digital converter for converting the analog imageinformation to the digital image information.
 18. The apparatus of claim9, wherein the plurality of device units each have the same pattern. 19.A method of detecting a defect on a substrate, the method comprising:irradiating a light on a substrate, wherein the substrate has aplurality of device units on a surface thereof, each device unitincluding a plurality of pixels; measuring image information for theplurality of pixels by sensing the light reflected by a surface of thesubstrate from the irradiating light; calculating a raw datum of atarget pixel by subtracting the image information of a correspondingpixel from the image information of the target pixel, wherein the targetpixel is a subject pixel for detecting a defect, wherein thecorresponding pixel is a pixel located in a first device unit andcorresponds to the target pixel, and wherein the first device unit islocated adjacent to a second device unit that includes the target pixel;presetting a threshold region including at least one pair of upper andlower limits; and comparing the raw datum with the threshold region. 20.The method of claim 19, further comprising marking the target pixel asdefective if the raw datum is within the threshold region.
 21. Themethod of claim 19, wherein the plurality of device units each have thesame pattern.
 22. The method of claim 19, wherein the irradiating lightincludes a short-wave light.
 23. The method of claim 22, wherein theshort-wave light includes an ultraviolet light.
 24. The method of claim19, wherein the image information includes binary digital information.25. The method of claim 24, wherein the image information represents alevel on a gray scale, wherein the gray scale is distinguishable by arelative density of black and white.
 26. The method of claim 25, whereinthe gray scale is divided into 256 different levels.
 27. The method ofclaim 19, further comprising displaying a defective pixel on a monitor.